In X-ray medical examinations the X-ray films of varying sizes conventionally packaged in cassettes are used. Generally, a film cassette is placed on the cassette tray which in turn is inserted into the grid cabinet. To screen secondary radiation from the film, stationary or moving lead line grids are used. Reciprocatory movement of the grids over the film prevents the grid lines from being photographed. It is desirable to have the film cassette automatically centered in the grid cabinet in such a way that the film is aligned with the X-ray beam. It is also desirable to sense the size of the film cassette and automatically communicate this information to a beam collimator for an automatic adjustment of the collimator's shutters in order to match the area of the X-ray beam with the film size. Linear or rotary potentiometers are used for communicating the information.
There are several cassette trays on the market which feature automatic centering of the film cassette and cassette size sensing for automatic collimation. All of the existing arts use a very similar centering system utilizing four jaws sliding from the center of the tray. Generally, means for cassette centering and means for cassette size sensing with sensing potentiometers are disposed in the tray, said tray being completely removable from the grid cabinet. Means for actuating grid to move over the film is traditionally disposed in the side of the cabinet and the grid motion is usually reciprocatory linear or damping sinusoidal.